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doc: add JSON and Go article

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Originally published on The Go Programming Language Blog, January 25, 2011.

http://blog.golang.org/2011/01/json-and-go.html

R=adg
CC=golang-dev
https://golang.org/cl/5846044
This commit is contained in:
Francisco Souza 2012-03-22 18:25:40 +11:00 committed by Andrew Gerrand
parent cec67568e9
commit 289a357104
10 changed files with 651 additions and 3 deletions
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1
doc/Makefile
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@ -11,6 +11,7 @@ RAWHTML=\
articles/go_concurrency_patterns_timing_out_moving_on.rawhtml\
articles/godoc_documenting_go_code.rawhtml\
articles/gobs_of_data.rawhtml\
articles/json_and_go.rawhtml\
articles/image_draw.rawhtml\
effective_go.rawhtml\
go1.rawhtml\

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doc/articles/json_and_go.html Normal file
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@ -0,0 +1,356 @@
<!--{
"Title": "JSON and Go",
"Template": true
}-->
<p>
JSON (JavaScript Object Notation) is a simple data interchange format.
Syntactically it resembles the objects and lists of JavaScript. It is most
commonly used for communication between web back-ends and JavaScript programs
running in the browser, but it is used in many other places, too. Its home page,
<a href="http://json.org">json.org</a>, provides a wonderfully clear and concise
definition of the standard.
</p>
<p>
With the <a href="/pkg/encoding/json/">json package</a> it's a snap to read and
write JSON data from your Go programs.
</p>
<p>
<b>Encoding</b>
</p>
<p>
To encode JSON data we use the
<a href="/pkg/encoding/json/#Marshal"><code>Marshal</code></a> function.
</p>
<pre>
func Marshal(v interface{}) ([]byte, error)
</pre>
<p>
Given the Go data structure, <code>Message</code>,
</p>
{{code "/doc/progs/json1.go" `/type Message/` `/STOP/`}}
<p>
and an instance of <code>Message</code>
</p>
{{code "/doc/progs/json1.go" `/m :=/`}}
<p>
we can marshal a JSON-encoded version of m using <code>json.Marshal</code>:
</p>
{{code "/doc/progs/json1.go" `/b, err :=/`}}
<p>
If all is well, <code>err</code> will be <code>nil</code> and <code>b</code>
will be a <code>[]byte</code> containing this JSON data:
</p>
<pre>
b == []byte(`{"Name":"Alice","Body":"Hello","Time":1294706395881547000}`)
</pre>
<p>
Only data structures that can be represented as valid JSON will be encoded:
</p>
<ul>
<li>
JSON objects only support strings as keys; to encode a Go map type it must be
of the form <code>map[string]T</code> (where <code>T</code> is any Go type
supported by the json package).
</li>
<li>
Channel, complex, and function types cannot be encoded.
</li>
<li>
Cyclic data structures are not supported; they will cause <code>Marshal</code>
to go into an infinite loop.
</li>
<li>
Pointers will be encoded as the values they point to (or 'null' if the pointer
is <code>nil</code>).
</li>
</ul>
<p>
The json package only accesses the exported fields of struct types (those that
begin with an uppercase letter). Therefore only the the exported fields of a
struct will be present in the JSON output.
</p>
<p>
<b>Decoding</b>
</p>
<p>
To decode JSON data we use the
<a href="/pkg/encoding/json/#Unmarshal"><code>Unmarshal</code></a> function.
</p>
<pre>
func Unmarshal(data []byte, v interface{}) error
</pre>
<p>
We must first create a place where the decoded data will be stored
</p>
{{code "/doc/progs/json1.go" `/var m Message/`}}
<p>
and call <code>json.Unmarshal</code>, passing it a <code>[]byte</code> of JSON
data and a pointer to <code>m</code>
</p>
{{code "/doc/progs/json1.go" `/err := json.Unmarshal/`}}
<p>
If <code>b</code> contains valid JSON that fits in <code>m</code>, after the
call <code>err</code> will be <code>nil</code> and the data from <code>b</code>
will have been stored in the struct <code>m</code>, as if by an assignment
like:
</p>
{{code "/doc/progs/json1.go" `/m = Message/` `/STOP/`}}
<p>
How does <code>Unmarshal</code> identify the fields in which to store the
decoded data? For a given JSON key <code>"Foo"</code>, <code>Unmarshal</code>
will look through the destination struct's fields to find (in order of
preference):
</p>
<ul>
<li>
An exported field with a tag of <code>"Foo"</code> (see the
<a href="/ref/spec#Struct_types">Go spec</a> for more on struct tags),
</li>
<li>
An exported field named <code>"Foo"</code>, or
</li>
<li>
An exported field named <code>"FOO"</code> or <code>"FoO"</code> or some other
case-insensitive match of <code>"Foo"</code>.
</li>
</ul>
<p>
What happens when the structure of the JSON data doesn't exactly match the Go
type?
</p>
{{code "/doc/progs/json1.go" `/"Food":"Pickle"/` `/STOP/`}}
<p>
<code>Unmarshal</code> will decode only the fields that it can find in the
destination type. In this case, only the Name field of m will be populated,
and the Food field will be ignored. This behavior is particularly useful when
you wish to pick only a few specific fields out of a large JSON blob. It also
means that any unexported fields in the destination struct will be unaffected
by <code>Unmarshal</code>.
</p>
<p>
But what if you don't know the structure of your JSON data beforehand?
</p>
<p>
<b>Generic JSON with interface{}</b>
</p>
<p>
The <code>interface{}</code> (empty interface) type describes an interface with
zero methods. Every Go type implements at least zero methods and therefore
satisfies the empty interface.
</p>
<p>
The empty interface serves as a general container type:
</p>
{{code "/doc/progs/json2.go" `/var i interface{}/` `/STOP/`}}
<p>
A type assertion accesses the underlying concrete type:
</p>
{{code "/doc/progs/json2.go" `/r := i/` `/STOP/`}}
<p>
Or, if the underlying type is unknown, a type switch determines the type:
</p>
{{code "/doc/progs/json2.go" `/switch v/` `/STOP/`}}
The json package uses <code>map[string]interface{}</code> and
<code>[]interface{}</code> values to store arbitrary JSON objects and arrays;
it will happily unmarshal any valid JSON blob into a plain
<code>interface{}</code> value. The default concrete Go types are:
<ul>
<li>
<code>bool</code> for JSON booleans,
</li>
<li>
<code>float64</code> for JSON numbers,
</li>
<li>
<code>string</code> for JSON strings, and
</li>
<li>
<code>nil</code> for JSON null.
</li>
</ul>
<p>
<b>Decoding arbitrary data</b>
</p>
<p>
Consider this JSON data, stored in the variable <code>b</code>:
</p>
{{code "/doc/progs/json3.go" `/b :=/`}}
<p>
Without knowing this data's structure, we can decode it into an
<code>interface{}</code> value with <code>Unmarshal</code>:
</p>
{{code "/doc/progs/json3.go" `/var f interface/` `/STOP/`}}
<p>
At this point the Go value in <code>f</code> would be a map whose keys are
strings and whose values are themselves stored as empty interface values:
</p>
{{code "/doc/progs/json3.go" `/f = map/` `/STOP/`}}
<p>
To access this data we can use a type assertion to access <code>f</code>'s
underlying <code>map[string]interface{}</code>:
</p>
{{code "/doc/progs/json3.go" `/m := f/`}}
<p>
We can then iterate through the map with a range statement and use a type switch
to access its values as their concrete types:
</p>
{{code "/doc/progs/json3.go" `/for k, v/` `/STOP/`}}
<p>
In this way you can work with unknown JSON data while still enjoying the
benefits of type safety.
</p>
<p>
<b>Reference Types</b>
</p>
<p>
Let's define a Go type to contain the data from the previous example:
</p>
{{code "/doc/progs/json4.go" `/type FamilyMember/` `/STOP/`}}
{{code "/doc/progs/json4.go" `/var m FamilyMember/` `/STOP/`}}
<p>
Unmarshaling that data into a <code>FamilyMember</code> value works as
expected, but if we look closely we can see a remarkable thing has happened.
With the var statement we allocated a <code>FamilyMember</code> struct, and
then provided a pointer to that value to <code>Unmarshal</code>, but at that
time the <code>Parents</code> field was a <code>nil</code> slice value. To
populate the <code>Parents</code> field, <code>Unmarshal</code> allocated a new
slice behind the scenes. This is typical of how <code>Unmarshal</code> works
with the supported reference types (pointers, slices, and maps).
</p>
<p>
Consider unmarshaling into this data structure:
</p>
<pre>
type Foo struct {
Bar *Bar
}
</pre>
<p>
If there were a <code>Bar</code> field in the JSON object,
<code>Unmarshal</code> would allocate a new <code>Bar</code> and populate it.
If not, <code>Bar</code> would be left as a <code>nil</code> pointer.
</p>
<p>
From this a useful pattern arises: if you have an application that receives a
few distinct message types, you might define "receiver" structure like
</p>
<pre>
type IncomingMessage struct {
Cmd *Command
Msg *Message
}
</pre>
<p>
and the sending party can populate the <code>Cmd</code> field and/or the
<code>Msg</code> field of the top-level JSON object, depending on the type of
message they want to communicate. <code>Unmarshal</code>, when decoding the
JSON into an <code>IncomingMessage</code> struct, will only allocate the data
structures present in the JSON data. To know which messages to process, the
programmer need simply test that either <code>Cmd</code> or <code>Msg</code> is
not <code>nil</code>.
</p>
<p>
<b>Streaming Encoders and Decoders</b>
</p>
<p>
The json package provides <code>Decoder</code> and <code>Encoder</code> types
to support the common operation of reading and writing streams of JSON data.
The <code>NewDecoder</code> and <code>NewEncoder</code> functions wrap the
<a href="/pkg/io/#Reader"><code>io.Reader</code></a> and
<a href="/pkg/io/#Writer"><code>io.Writer</code></a> interface types.
</p>
<pre>
func NewDecoder(r io.Reader) *Decoder
func NewEncoder(w io.Writer) *Encoder
</pre>
<p>
Here's an example program that reads a series of JSON objects from standard
input, removes all but the <code>Name</code> field from each object, and then
writes the objects to standard output:
</p>
{{code "/doc/progs/json5.go" `/package main/` `$`}}
<p>
Due to the ubiquity of Readers and Writers, these <code>Encoder</code> and
<code>Decoder</code> types can be used in a broad range of scenarios, such as
reading and writing to HTTP connections, WebSockets, or files.
</p>
<p>
<b>References</b>
</p>
<p>
For more information see the <a href="/pkg/encoding/json/">json package documentation</a>. For an example usage of
json see the source files of the <a href="/pkg/net/rpc/jsonrpc/">jsonrpc package</a>.
</p>

2
doc/docs.html
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@ -113,7 +113,7 @@ Guided tours of Go programs.
<h4>Packages</h4>
<ul>
<li><a href="http://blog.golang.org/2011/01/json-and-go.html">JSON and Go</a> - using the <a href="/pkg/encoding/json/">json</a> package.</li>
<li><a href="/doc/articles/json_and_go.html">JSON and Go</a> - using the <a href="/pkg/encoding/json/">json</a> package.</li>
<li><a href="/doc/articles/gobs_of_data.html">Gobs of data</a> - the design and use of the <a href="/pkg/encoding/gob/">gob</a> package.</li>
<li><a href="/doc/articles/laws_of_reflection.html">The Laws of Reflection</a> - the fundamentals of the <a href="/pkg/reflect/">reflect</a> package.</li>
<li><a href="http://blog.golang.org/2011/09/go-image-package.html">The Go image package</a> - the fundamentals of the <a href="/pkg/image/">image</a> package.</li>

88
doc/progs/json1.go Normal file
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@ -0,0 +1,88 @@
// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
import (
"encoding/json"
"log"
"reflect"
)
type Message struct {
Name string
Body string
Time int64
}
// STOP OMIT
func Encode() {
m := Message{"Alice", "Hello", 1294706395881547000}
b, err := json.Marshal(m)
if err != nil {
panic(err)
}
expected := []byte(`{"Name":"Alice","Body":"Hello","Time":1294706395881547000}`)
if !reflect.DeepEqual(b, expected) {
log.Panicf("Error marshalling %q, expected %q, got %q.", m, expected, b)
}
}
func Decode() {
b := []byte(`{"Name":"Alice","Body":"Hello","Time":1294706395881547000}`)
var m Message
err := json.Unmarshal(b, &m)
if err != nil {
panic(err)
}
expected := Message{
Name: "Alice",
Body: "Hello",
Time: 1294706395881547000,
}
if !reflect.DeepEqual(m, expected) {
log.Panicf("Error unmarshalling %q, expected %q, got %q.", b, expected, m)
}
m = Message{
Name: "Alice",
Body: "Hello",
Time: 1294706395881547000,
}
// STOP OMIT
}
func PartialDecode() {
b := []byte(`{"Name":"Bob","Food":"Pickle"}`)
var m Message
err := json.Unmarshal(b, &m)
// STOP OMIT
if err != nil {
panic(err)
}
expected := Message{
Name: "Bob",
}
if !reflect.DeepEqual(expected, m) {
log.Panicf("Error unmarshalling %q, expected %q, got %q.", b, expected, m)
}
}
func main() {
Encode()
Decode()
PartialDecode()
}

42
doc/progs/json2.go Normal file
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@ -0,0 +1,42 @@
// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
import (
"fmt"
"math"
)
func InterfaceExample() {
var i interface{}
i = "a string"
i = 2011
i = 2.777
// STOP OMIT
r := i.(float64)
fmt.Println("the circle's area", math.Pi*r*r)
// STOP OMIT
switch v := i.(type) {
case int:
fmt.Println("twice i is", v*2)
case float64:
fmt.Println("the reciprocal of i is", 1/v)
case string:
h := len(v) / 2
fmt.Println("i swapped by halves is", v[h:]+v[:h])
default:
// i isn't one of the types above
}
// STOP OMIT
}
func main() {
InterfaceExample()
}

73
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@ -0,0 +1,73 @@
// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
import (
"encoding/json"
"fmt"
"log"
"reflect"
)
func Decode() {
b := []byte(`{"Name":"Wednesday","Age":6,"Parents":["Gomez","Morticia"]}`)
var f interface{}
err := json.Unmarshal(b, &f)
// STOP OMIT
if err != nil {
panic(err)
}
expected := map[string]interface{}{
"Name": "Wednesday",
"Age": float64(6),
"Parents": []interface{}{
"Gomez",
"Morticia",
},
}
if !reflect.DeepEqual(f, expected) {
log.Panicf("Error unmarshalling %q, expected %q, got %q", b, expected, f)
}
f = map[string]interface{}{
"Name": "Wednesday",
"Age": 6,
"Parents": []interface{}{
"Gomez",
"Morticia",
},
}
// STOP OMIT
m := f.(map[string]interface{})
for k, v := range m {
switch vv := v.(type) {
case string:
fmt.Println(k, "is string", vv)
case int:
fmt.Println(k, "is int", vv)
case []interface{}:
fmt.Println(k, "is an array:")
for i, u := range vv {
fmt.Println(i, u)
}
default:
fmt.Println(k, "is of a type I don't know how to handle")
}
}
// STOP OMIT
}
func main() {
Decode()
}

45
doc/progs/json4.go Normal file
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@ -0,0 +1,45 @@
// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
import (
"encoding/json"
"log"
"reflect"
)
type FamilyMember struct {
Name string
Age int
Parents []string
}
// STOP OMIT
func Decode() {
b := []byte(`{"Name":"Bob","Age":20,"Parents":["Morticia", "Gomez"]}`)
var m FamilyMember
err := json.Unmarshal(b, &m)
// STOP OMIT
if err != nil {
panic(err)
}
expected := FamilyMember{
Name: "Bob",
Age: 20,
Parents: []string{"Morticia", "Gomez"},
}
if !reflect.DeepEqual(expected, m) {
log.Panicf("Error unmarshalling %q, expected %q, got %q", b, expected, m)
}
}
func main() {
Decode()
}

31
doc/progs/json5.go Normal file
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@ -0,0 +1,31 @@
// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
import (
"encoding/json"
"log"
"os"
)
func main() {
dec := json.NewDecoder(os.Stdin)
enc := json.NewEncoder(os.Stdout)
for {
var v map[string]interface{}
if err := dec.Decode(&v); err != nil {
log.Println(err)
return
}
for k := range v {
if k != "Name" {
delete(v, k)
}
}
if err := enc.Encode(&v); err != nil {
log.Println(err)
}
}
}

14
doc/progs/run
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@ -51,7 +51,15 @@ gobs="
gobs2
"
all=$(echo $defer_panic_recover $effective_go $error_handling $law_of_reflection $c_go_cgo $timeout $gobs slices go1)
json="
json1
json2
json3
json4
json5
"
all=$(echo $defer_panic_recover $effective_go $error_handling $law_of_reflection $c_go_cgo $timeout $gobs $json slices go1)
for i in $all; do
go build $i.go
@ -79,5 +87,9 @@ testit eff_sequence '^\[-1 2 6 16 44\]$'
testit go1 '^Christmas is a holiday: true Sleeping for 0.123s.*go1.go already exists$'
testit interface2 "^type: float64$"
testit json1 "^$"
testit json2 "the reciprocal of i is"
testit json3 "Age is int 6"
testit json4 "^$"
rm -f $all "$TMPFILE"

2
src/pkg/encoding/json/encode.go
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@ -6,7 +6,7 @@
// RFC 4627.
//
// See "JSON and Go" for an introduction to this package:
// http://blog.golang.org/2011/01/json-and-go.html
// http://golang.org/doc/articles/json_and_go.html
package json
import (